1. Field of the Invention
The present invention relates to a SQUID (Superconducting Quantum Interference Device) fluxmeter.
2. Description of the Related Art
The SQUID fluxmeter is the most sensitive of all fluxmeters. It is utilized, in particular, for the measurement of magnetic fields emitted from organisms. From the viewpoint of clinical application, there is a demand for a SQUID fluxmeter of higher quality.
FIG. 8 is a circuit diagram showing a conventional SQUID fluxmeter with a built-in superconducting feedback circuit. The SQUID fluxmeter shown consists of a SQUID sensor and a feedback circuit.
The SQUID sensor consists of a pick-up coil 10, a SQUID 12 and an AC bias current source 14.
The pick-up coil 10, which is of a primary differential type adapted to compensate for geomagnetism, is magnetically connected to the SQUID 12. The SQUID 12 consists of a superconducting loop including Josephson devices J1 and J2. The SQUID 12 is supplied with an AC bias current as shown in FIG. 9(A) from the AC bias current source 14.
The feedback circuit consists of a write gate 16 having a SQUID 18 and a magnetic coupling coil 20, a superconducting accumulation loop 22 and a feedback coil 24. One end of the magnetic coupling coil 20 is connected to the output node of the SQUID 12, and the other end thereof is connected to the input node of the SQUID 18. The SQUID 18, the superconducting accumulation loop 22 and the feedback coil 24 are connected together in a loop-like fashion. A feedback magnetic flux is supplied to the SQUID 12 through a feedback coil 24.
In the above-described construction, when the SQUID 12 receives an input magnetic flux through the pick-up coil 10, a write pulse as Shown in FIG. 9(B) is supplied from the SQUID 12 to the magnetic coupling coil 20. At the rise and fall of each write pulse, Josephson junctions J3 and J4 are switched on, respectively, whereby a fluxoid quantum is added to the superconducting accumulation loop 22. By reason of the direction of the fluxoid quantum added to the superconducting accumulation loop 22 in the case of a positive write pulse is reverse to that of a negative write pulse, a magnetic flux .psi. as shown in FIG. 9(C) is stored in the superconducting accumulation loop 22, which functions as an up-down counter. A magnetic flux in proportion to the accumulated magnetic flux .psi. is supplied to the SQUID 12 through the feedback coil 24 in such a way to cancel the input magnetic flux from the pick-up coil 10, and a voltage pulse train is output from the SQUID 12 until the magnetic flux in the SQUID 12 is reduced to zero.
This pulse train is also supplied to a counter 32 placed on the room-temperature side and counted there, the count value C1 of the counter 32 indicating a measured magnetic flux in the SQUID 12 through the pick-up coil 10.
Such a SQUID fluxmeter can be manufactured in a one-chip structure, exclusive of the AC bias current source 14. Thus, magnetic-flux-distribution measurement is possible with a number of such SQUID fluxmeters arranged. Such a SQUID fluxmeter advantageously leads to a reduction in the number of cables connecting from the circuit on the room-temperature side to the SQUID chips on the low temperature side, thereby reducing the heat flow from the room-temperature side to the low-temperature side. Further, it also leads to a reduction in crosstalk between cables.
However, the above conventional SQUID fluxmeter has a problem in that only a fluxoid quantum at most can be written to the superconducting accumulation loop 22 with respect to each of the positive and negative half-cycles of AC bias current. Since the magnetic flux feedback to the SQUID 12 is the first order lag, the response speed .DELTA..psi..multidot.f of the magnetic-flux feedback through the feedback coil 24 to the SQUID 12 in response to the input magnetic flux is low, where .DELTA..psi. is the increase of feedback magnetic flux to the SQUID 12 when one fluxoid quantum is added to the superconducting accumulation loop 22, and f indicates the frequency of the AC bias current.
When the frequency f is made too high so as to increase the response speed, the oscillating current induced to the pick-up coil by the switching of the SQUID 12 in a half-circle of the AC bias current induces back to the pick-up coil 10 in the next half-circle of the AC bias current, with the result that the upper limit of the frequency f is restricted to a value determined by the circuit constants of the SQUID sensor and the pick-up coil. Further, when the unit feedback magnetic flux .DELTA..psi. is made too large, the quantization noise of the SQUID sensor increases due to the feedback magnetic flux.
Thus, when the input magnetic flux has greatly changed in a short period of time, it is impossible to measure the magnetic flux accurately. Depending upon the object of measurement, it is necessary to increase the response speed at the expense of sensitivity.